Recent progress achieved in the field of semiconductor lasers opens up advantageous prospects for future transmission systems by using optical fibers. The good monochromaticity of distributed feedback lasers (DFB for short) enables the capacity of connections to be considerably enhanced by transmitting several wavelength on the same fiber by optical multiplexing. The distance between the repeaters can also be increased by using the coherent detection technique. This is of course a technique in which the light transmitted by the optical fiber is combined on the same detector with the light originating from a laser of the same wavelength known as a "local oscillator".
However, one of the crucial problems raised by these applications is the wavelength dispersion of the lasers originating from the same semiconductor plate or two different plates. The dispersions observed up to date on DFB lasers are several tens of Angstroms (20 and 30). Filtering operations are therefore necessary to make the emission wavelength of the source coincide with that of the optical multiplexers or demultiplexers, or else to make the wavelength of the source coincide with that of the local oscillator for coherent systems.